The proper operation of the seeding machines is supervised with a seed control system. The key elements of the seed control system include the seed sensors by means of which dispensing of the seeds can be supervised. The present invention primarily relates to seed sensors used in pneumatic grain seeding machines (wherein the seeds are conveyed to the soil by means of air), wherein the seed sensors provide real-time information on the accuracy of the seeding process and the faults of the seeding process for the operator of the seeding machine.
In the present state of the art numerous solutions are used to detect seeds during the seeding process. The most common solution is the use of so called seed detection sensors (seed sensors). The seed sensors are placed in the flow path of the seeds, therefore the seeds get to the soil with passing through the seed sensors. During seed control, the seed sensors have the function to determine whether or not the seeding machine is seeding, and to determine how many seeds are dispensed by the seeding machine within a specified time period. The seed sensors have the further function to determine if a seed conveying pipe is blocked. Various technical solutions may be applied to provide the above functions. The most common solutions are based on optical principles, but there also exist seed sensors operating with radio waves (radar), micro wave or on acoustical basis.
Most of the optical seed sensors operate using the principle of the optogate. In this solution light sources (transmitters) and light detectors (receivers) are arranged opposite to each other. Typically, the light sources are light emitting diodes (LED), whereas the light detectors are respective phototransistors or photodiodes, in general. The key point of the operation of the optogate-type seed sensors is that an object (such as a seed in the present case) passing between a transmitter (e.g. LEDs) and a receiver, produces a shadow on the receiver side, which, in turn, generates a signal in the signal processing circuit of the receiver.
The optogate-type seed sensors are basically classified into two types according to the control scheme of the light sources. The first type includes seed sensors in which the light sources operate continuously and with constant light intensity in the course of time. Regarding the operation (i.e. the control of the light sources and the signal processing) this solution is the simpler one and therefore it is also more common. The other group includes those optogate-type seed sensors in which the light intensity of the light sources is controlled by periodic signals, for example it is modulated by a constant frequency square pulse. In this case detection of the seeds takes place by virtue of the examination of the distortion of the signal detected on the receiver side. The number of the light sources and the number of the light detectors are selected depending on a specific place of the application, the shape and the dimensions of the seed sensing zone. The size of the seeds passing through a seed sensor may be even as small as 1 mm for small seeds (e.g. cabbage, rape), but for larger seeds the size may reach even 20 mm (e.g. horse-bean). The flow rate of the seeds passing through the seed sensors mostly depends on the type of the seeding machine since the currently used seeding machines dispense the seeds in two different ways. In one type of the machines, the seeds are mechanically dropped (these are the so called seed planters), wherein the seeds reach the soil due to the gravitation. In this case the flow rate of the seeds is regarded low. In the other type of seeding machines, the seeds are dispensed by means of air (these are the so called pneumatic seeding machines), in which, due to an intensive air stream, the flow rate of the seeds are comparatively higher. Certain types of seeds are dispensed in large doses, whereby the number of seeds passing through a seed sensor may reach even 300 seeds/sec, for example at wheat grain seeding. This case is called large dose dispensing.
Depending on the place of use, in view of the above features, the optical seed sensors may be classified into two major groups, one including the seed sensors used for seed planting, and the other one including the seeds sensor used for pneumatic seeding.
According to the present state of the art, the seed sensors used for seed planting are the most advanced devices in general as they detect seeds of both small and large sizes at high precision and provide accurate information on the seeding process. During seed planting the seed planter dispenses the seeds individually, therefore a seed sensor has to detect and count the seeds that are passing through the seed sensor successively. The seed sensors used for seed planting are adapted to control the dispensed amount of seeds and the spatial distribution of the dispensed seeds in real time. Such a seed sensor is disclosed, for example, in the U.S. Pat. No. 8,843,281. In this solution, along the side of the sensor, light sources are equidistally arranged on either side of a detection chamber and light detectors are equidistally arranged on the other side of the detection chamber opposite to the light sources, wherein the distances between the light sources and the light detectors are adjusted so that no dead space can develop in the seed sensor. Due to the high number of light detectors on the receiver side, the sensors can determine even the size of the seeds. The asymmetric arrangement used in this solution is beneficial to eliminate the dead space but it has an adverse effect to the physical dimensions of the seed sensor. Furthermore, the commonly used LED light sources do not provide uniform light in the entire range of their illumination angle, therefore it is possible that within the overall detection chamber of the seed sensor there is no homogenous light. The homogenous light, i.e. the homogenity of the light intensity, within the seed sensor is necessary for properly detecting the seeds of various sizes at various light intensities.
The accuracy of the seed sensors therefore depends on the sensitivity of the sensor which is recommended to be adjusted so that it suits to the type of the seeds for achieving the highest possible accuracy. This object is solved by a seed sensor described in the document WO 2014/035949, in which a plurality of light sources (LEDs) provide homogenous light intensity within the detection chamber. However the intensity of the light sources (LEDs) arranged in the middle and in the periphery of the seed sensor is not uniform because of the light beams reflected from the walls, therefore different light intensity values are used for those light sources which are arranged along the peripheries and for those which are in the middle of the seed sensor. Due to this solution homogenous light can be produced in the seed sensor which allows to properly detect the seeds passing through the seed sensor at any point within the seed sensor. In this solution the light intensity of the light sources is constant. Sensitivity of the seed sensor may be set by adjusting the output amplification of the receiver (photovoltaic sensor). In this circuit the sensitivity can be adjusted to a few levels. The detection process of the sensor is assisted by an optical unit arranged on the receiver side, wherein the light beams of the transmitters (LEDs) are directed to be parallel by using Fresnel-lens, whereby the interference between them is reduced and the seeds arriving at the same time in parallel can be distinguished from each other more easily, which allows a more precise seed detection.
The way of seeding of a pneumatic grain seeding machine substantially differs from the uniform dispensing of the seeds carried out by a seed planter (wherein the seeds are planted individually). The pneumatic grain seeding machines do not dispense the seeds individually, one-by-one. One of the reasons for it is that in case of less invasive plant cultures (e.g. wheat) it is not so important to keep an exact distance between the plants as it is required for the invasive plant cultures (e.g. maize) seeded by seed planters. That is why the seeds in a pneumatic grain seeding machine move across the seed sensors at a higher speed and non-uniformly, therefore the seeds often move through the detection chamber close to each other in parallel, with one masking another one.
Because of the above described features of the pneumatic grain seeding, the seed sensors used in these machines can only detect the “seeding” or “not seeding” state of the seed conveying pipes of the seeding machine, while they are not capable of counting the number of seeds passing through the seed sensor. The most common method of detecting the “seeding” or “not seeding” state is that in case the detected number of seeds passing through a seed conveying pipe decreases below the lowest expected frequency value (e.g. measured in the number of seeds per second), then the seed sensor indicates a “not seeding” state. Besides the “not seeding” state, the known seed sensors are also capable of determining the blockage of a seed conveying pipe since according to the experiences, when the expected number of seeds per second decreases at least to its half value in a seed conveying pipe, it is caused by the blockage of the seed conveying pipe at high probability. These seed sensors, however, are capable of detecting the expected number of seeds only after a calibration procedure in which the number of signals generated by the detectable seeds is counted and the expected number of seeds is then determined. The inaccuracy of the seed sensors of the known pneumatic grain seeding machines depends on the amount of seeds dispensed by the machine, whereas the dispensed amount of seeds depends on the speed of the machine. With dispensing at higher intensity, when the seeds pass through the sensors at a relatively high speed and with a non-uniform spatial and temporal distribution, the known seed sensors are not able to determine the exact number of the seeds, i.e. they detect the passage of less seeds than the real number thereof, therefore the detection of blockage of a seed conveying pipe may be missed in many cases. A further problem is that the dispensing of the pneumatic seeding machines may also vary row by row, and in case of seeding with a variable output rate, the dispensed amount of the seeds may also change area by area, therefore the maximum of the dispensing rate of seeds should also be adjusted so that blockage can be detected.